Noise control and moisture management constitute two rapidly growing economic and public policy concerns for the construction industry. Areas with high acoustical isolation (commonly referred to as ‘soundproofed’) are requested and required for a variety of purposes. Apartments, condominiums, hotels, schools and hospitals all require rooms with walls, ceilings and floors that reduce the transmission of sound thereby minimizing, or eliminating, the disturbance to people in adjacent rooms. Soundproofing is particularly important in buildings adjacent to public transportation, such as highways, airports and railroad lines. Additionally theaters, home theaters, music practice rooms, recording studios and others require increased noise abatement. Likewise, hospitals and general healthcare facilities have begun to recognize acoustical comfort as an important part of a patient's recovery time. One measure of the severity of multi-party residential and commercial noise control issues is the widespread emergence of model building codes and design guidelines that specify minimum Sound Transmission Class (STC) ratings for specific wall structures within a building. Another measure is the broad emergence of litigation between homeowners and builders over the issue of unacceptable noise levels. To the detriment of the U.S. economy, both problems have resulted in major builders refusing to build homes, condos and apartments in certain municipalities; and in widespread cancellation of liability insurance for builders.
In addition to the issue of noise control, moisture management is an equally important construction industry concern. The problems associated with excessive moisture in building wall cavities and the resulting mold growth, have driven a national outcry over unhealthy buildings and poor indoor air quality. Walls between areas of differing temperature are the primary structures for these problems. Preventing condensation is of particular importance with regard to the exterior walls of a home or other buildings, where temperature extremes are likely to be greater than between interior walls. Wetting of exterior building surfaces and rainwater leaks are major causes of water infiltration, but so is excessive indoor moisture generation. Moisture may be present within a structure due to occupancy and use by humans, use of wet materials during construction, air leaks, or transportation by external wall materials. Moisture accumulates when the rate of moisture entry into an assembly (or its generation from within, such as cooking, bathing, painting and other activities) exceeds the rate of moisture removal. The flow of moisture and water vapor is controlled via a vapor resistive membrane or material termed a vapor retarder. A vapor retarder is defined as a membrane or panel having a water vapor transmission rate of less than one perm. See paragraph [0007] below for the definition of a “perm”. Conversely, an air retarder is a membrane or panel having a water vapor transmission rate of less than five perms. High perm values ensure ready transmission of water vapor. Common vapor retarders include polyethylene films and foil.
It is obvious that the problem is compounded when a single wall or structure needs to effectively both abate high noise levels and allow for proper water vapor transport without leakage.
For example, a conflict in the two requirements is evident in the case of a new class of damped drywall panels designed for soundproofing. These panels are an improvement to traditional noise control materials such as resilient channels, double stud framing or multiple drywall layers because they offer a materials and labor savings over traditional techniques. However, they have major shortcomings with regard to proper water vapor transport. These damped drywall panels incorporate a continuous film of polymer and therefore act as a vapor retarder. The Department of Energy (DOE) and the American Society of Heating, Refrigeration, and Air-Conditioning Engineers (ASHRAE) and other building science organizations have established recommended wall designs and the proper location of a vapor retard within the wall. These designs are dependant upon the local climate. In cooling-dominated climates, it is recommended that a vapor retarder be installed on the exterior of the thermal insulation. In mixed zones—climates with both significant heating and cooling requirements—design recommendations suggest the omission of the vapor retarder altogether. If these guidelines are not observed, the structure is at risk of allowing water vapor condensation within the wall cavity.
If these materials are used in conjunction with a typical vapor barrier serious problems may develop. A wall having a vapor barrier on both sides of the structure should be avoided so that moisture will not be trapped and condensed, leading to mold or wood rot. Traditional methods have used one or more layers of typical drywall (which allows for ready transport of water vapor) on the interior side of the wall structure and a vapor barrier on the exterior side of the wall structure. Such a design diminishes the moisture-related problems, however the typical wall structure provides little sound attenuation.
A figure of merit for the sound reducing qualities of a material or method of construction is the material's Sound Transmission Class (STC). The STC numbers are ratings Which are used in the architectural field to rate partitions, doors and windows for their effectiveness in blocking sound. The rating assigned to a particular partition design as a result of acoustical testing represents a best fit type of approach to a set of curves that define the sound transmission class. The test is conducted in such a way as to make it independent of the test environment and gives a number for the partition only. The STC measurement method is defined by ASTM E90 laboratory test for sound measurements obtained in ⅓ octave bands, and ASTM E413 for calculating STC numbers from the sound transmission loss data for a structure. These standards are available on the Internet at http://www.astm.org.
A figure of merit for the measurement of the transport of water vapor, by a material or method of construction, is its permeance, or “perms”. One perm is defined as the transport of one grain of water per square foot of exposed area per hour with a vapor pressure differential of 1-inch of mercury (Hg). Vapor pressure is a function of the temperature and relative humidity (RH) of the air to which a test structure is exposed, and may be found in many standard data tables. The vapor pressure at any certain RH is found by the product of the RH and the vapor pressure for saturated air at a certain temperature. For example, at 70 degrees Fahrenheit the saturated vapor pressure is 0.7392 inches Hg and the vapor pressure at fifty percent RH is 0.3696 inches Hg. The testing methodology varies depending upon the subject material. Data presented herein after was taken using the ASTM E96 “dry cup” method. Further information may be found on the Internet at http://www.astm.org.
Accordingly, what is needed is a new material and a new method of construction to reduce the transmission of sound from a given room to an adjacent area while simultaneously allowing the controlled transport of moisture from an area of higher concentration to an area of lower concentration.